The present invention relates to the anaerobic biological conversion of biomass such as industrial sludges, slurried refuse and agricultural residues to combined heat and power (CHP). More particularly, this invention relates to a system, a digestion apparatus, and a process for effecting the rapid and complete anaerobic hydrolysis of biomass comprising wet organic matter, and the ultimate conversion of soluble and gaseous byproducts to methane and, ultimately, to useful thermal or electrical energy.
Anaerobic digestion (AD) is a technology having three main disadvantages that result in its rarely being considered for energy production. The principal disadvantage stems from the fact that the reduction in volatile organic solid material is frequently far from complete. The results depend on the substrate, but incomplete conversion is typical of systems in which water is the sole plasticizing agent and hydrolytic pretreatment is not employed.
The second disadvantage is the long hydraulic residence time or the length of time the liquid must stay in the digestion system to complete the transformation of the slowest metabolizing materials. Unless the digester is heated, the residence time in the digester can be very long, as much as 40 to 60 days. The third disadvantage is that conventional digestion is prone to failure caused by three types of overloadsxe2x80x94organic, hydraulic, and toxicxe2x80x94that can result in the disruption of gas production. In this case, the digestion system must be taken out of service, the digester tank(s) cleaned out, and the system restarted. The environmental effects of such failures can be serious, particularly when other facilities for treatment are unavailable and raw sewage or untreated sludge must be disposed directly to the environment.
Incomplete solids hydrolysis is caused by two problems. First, biomass is a mixture of colloidal and particulate constituents that have very different hydrolysis rates. Some organic constituents are metabolized and degrade more rapidly than others. For example, common soluble chemical intermediates such as acetic acid and glucose, as contained in sugar waste waters, are constituents that degrade rapidly. On the other hand, constituents that degrade slowly or not at all include particulate and colloidal materials, such as proteins, fats, vegetable oils, tallow, bacterial and yeast cell walls, lignin and cellulose. Accordingly, the hydrolysis of the most resistant organic fraction becomes the efficiency limiting step since complete degradation can take place only after hydrolysis of all the insoluble constituents"" has occurred. In conventional AD, the accumulation of unwanted digestion products wastes reactor space. The economic use of reactor space dictates that the diverse symbiotic bacterial mass and undigested material be efficiently captured and the spent materials be efficiently removed to ensure the hydrolysis of the slowest metabolizing materials. Incomplete hydrolysis and solids accumulation in conventional AD systems is generally responsible for the poor performance for these systems.
Conventional AD systems are prone to failure, and operational control has been problematic. Different biomass substrates can have very different degradative characteristics, or different ratios of easily degradable material to refractory organic material. This limits AD systems to one particular substrate and to a small loading range to insure continuous uninterrupted operation. The loading limits are determined by trial and error experimentation. To insure that the operation stays within the limits of digestion, AD operators monitor total gas production supplemented with intermittent analysis of pH, alkalinity and volatile solids testing of the mixed liquor. Error correction is accomplished by manually adjusting the flow rate and solids loading. However, the warning signs of imminent failure usually come too late. This haphazard process control methodology is insufficient to guarantee uninterrupted operation needed for energy production purposes.
Various approaches have been proposed for overcoming the shortcomings of conventional AD systems. The disclosures of all of the patents discussed in the paragraphs that follow are incorporated herein by reference.
Many investigators have shown the value in recycling solids between two reactors to maintain high substrate and bacterial enzyme concentrations. For example, in U.S. Pat. No. 4,559,142 to Morper, it is recognized that it is economically advantageous to process the more slowly hydrolyzable material in a second reactor, separate from a reactor where the more rapidly hydrolyzed material is treated. Other investigators have recognized the benefit of maintaining the second reactor at a higher temperature to increase hydrolysis of slower hydrolyzing materials. However, these patents do not teach the control of temperature and pressure cycling on the second digester to improve both the rate and completeness of the digestion process.
In U.S. Pat. Nos. 5,015,384 and 5,670,047 to Burke, mechanical or chemical enhanced mechanical means are used to thicken and separate the partially digested particulate constituents from the effluent stream and recycle the particulates back to the digester, saving substrate and bacterial enzymes to further the hydrolysis. In subsequent U.S. Pat. No. 6,113,786, Burke recognized the advantage of mechanically removing inorganic solids from the reacting medium in order to preserve reactor space for the partially digested organic solids. The Burke patents, however, do not suggest a process design that promotes in-reactor thickening while digestion and advanced hydrolysis is ongoing, nor do they teach the intermittent separation and removal of inorganic solids via short term gas expulsion during reactor blowdown.
Investigators have described the need to improve the digestibility or hydrolysis of wet biomass. In U.S. Pat. No. 5,785,852 to Rivard et al. is proposed an elaborate pretreatment scheme using a pressurized thermochemical and mechanical processes to liquefy 44-66% of the sludge solid prior to digestion. The resulting soluble mixture is then amenable to conventional AD and the inorganic solids washout with the system effluent. The Rivard patent does not suggest the use of in-reactor pressure swing digestion technique to improve the digestibility and hydrolysis of biomass, nor does it teach the reaction of gas plasticization via pressure cycling to disrupt the sludge integrity and enable the advanced hydrolysis of refractory particles.
In U.S. Pat. No. 4,642,187 and related U.S. Pat. Nos. 4,401,565 and 4,375,412, the inventor of the present application described a system for separating and routing the slowly hydrolyzable material into a second reactor gas-solid suspension. Anaerobic bacteria are contacted with an influent slurry containing solid organic material, refractory organic material, and undissolved inorganic material, which are captured and recycled between the reactors in a closed loop to achieve a high conversion of the organic material to gaseous products, including methane. It was observed that pressure cycling in this second reactor facilitates the rapid anaerobic breakdown of refractory particulates, as measured by maximum volatile solids reduction and total gas production.
This basic approach has enabled rapid refractory solids hydrolysis with the continuous maintenance of a high bacterial concentration within the system reactors to attain nearly full solids conversion. These factors have allowed a significant reduction in digestion tankage and thus have improved the economic factors involved in energy production. U.S. Pat. No. 4,642,187 and its related patents do not teach the method of tuning the pressure swing program, the nature of the second stage reaction, or a variety of modifications needed to enable a wide array of bioenergy applications. Neither do they teach or suggest an apparatus or process of driving the hydrolysis of refractory solids substantially to completion via gas plasticization.
Furthermore, in U.S. Pat. No. 4,642,187, the inventor of the subject application described a method and baffle assembly in the first stage concentrator tank that diverts the flow to assist in stripping the gas, thereby facilitating solid-liquid separation and sedimentation. Solids undergoing active digestion often have gas bubbles adhered to them, which changes their specific gravity and settling characteristics. However, if a gasified particle contacts the gas phase during a free-fall cascade, the gases easily transfer from the solid to the gas phase, essentially degassing the solid and enabling the solid to concentrate by gravitational settling. Passing the gasified particles through a submerged gas volume is a minimum cost method of accomplishing effective solids-liquid separation and thickening in anaerobic environments. U.S. Pat. No. 4,642,187, however, does not teach or suggest the additional use of submerged gas volume to collect a gas volume needed to offset volume losses occurring in the second stage vacuum digester.
The present invention is directed to an improved digester apparatus for converting wet carbonaceous biomass materials to biogas comprising gaseous organic fuel. The apparatus comprises a digestion unit operating at a controlled temperature and having a concentrator component and a pressure swing component each containing anaerobic bacteria and means for conveying aqueous slurried biomass from a biomass source to the concentrator component, and for removing the biogas from the concentrator component. The apparatus also includes means for conveying concentrated aqueous biomass from the concentrator component to the pressure swing component and for conveying digested aqueous biomass from the pressure swing component to the concentrator component, thereby forming a closed loop between the concentrator and pressure swing components. The apparatus further includes means for removing waste solids from the pressure swing component and a pressure swing pump for controlling the pressure within the pressure swing component in a cycle comprising a first phase having a first time duration at a sub-atmospheric first pressure and a second phase having a second time duration and a second pressure at or above atmospheric pressure.
The improvement comprises: inclusion in the apparatus of a programmable computer provided with a database comprising data relating previously determined biomass-biogas conversion for biomass materials of varying compositions to values for the first and second, respectively, pressures and time durations of the first and second phases, wherein the computer operates to continuously monitor the pressure of biogas in the pressure swing component and adjust the cycle of the pressure swing pump to optimize biogas conversion of biomass from the biomass source.
This invention, which extends the capability the anaerobic digestion (AD) to substantially complete the transformation of wet biomass to methane gas and energy products, makes use of an ordinary mixed culture of anaerobes, or a pure culture or a genetically modified organism(s) (GMO) to extract the maximum amount of energy from biomass immersed in water. Fully recovering the mass energy content under water is dependent on completing microbial mass hydrolysis. A unique pressure swing method that cycles between pressurization-depressurization and is computer controlled to optimize system efficiency and reliability is capable of superior hydrolysis performance in anaerobic digestion, particularly of difficultly degradable materials such as cellulose.
Accordingly, it is an object of the present invention to provide an improved anaerobic cyclic digestion system for converting wet organic biomass materials such as sewage sludge to useful energy by optimizing the process through computer control.
It is a further object of the invention to utilize a programmable computer to control the cyclic digestion process, monitor the process for conditions leading to reduced performance, and take corrective action to maintain optimum operation.
It is a still further object of the invention to utilize gas plasticization to improve the second stage hydrolysis and digestion in the influent material fed to the system.
It is a further object of the invention to utilize a computer to monitor the anaerobic sludge digestion process for indication of insufficient plasticization and take corrective action to improve gas adsorption and penetration and optimize the second reactor hydrolysis rate.
It is a further object of the invention to provide an improved method for continuously and automatically correcting for loss or gain in system hydraulic volume to accommodate in-reactor biomass thickening during active continuous digestion by providing a submerged gas volume that is filled with gas production in the first reactor and subsequently used to offset the volume loss in the second reactor.
It is a further object of the invention to provide an improved method for continuous and automatic solids capture by providing a submerged gas volume at the top of the first reactor.
It is a further object of the present invention to provide an improved method for removing inorganic solids, thereby preventing the buildup of inorganic materials and maintaining system digestion capacity during active continuous digestion.
It is a further object of the present invention to provide means for routing the ammonia-containing gas stream to the pressure side of the digestion system and thereby scrub the ammonia gas from the gas phase.
It is a still further object of the present invention to provide a means for scrubbing hydrogen sulfide gas, an undesirable combustion fuel contaminant that corrodes metals and produces sulfur dioxide, whose combustion leads to photochemical smog, from the biogas stream.